One Dimensional Motion

Time and Distance

By one dimension we mean that the body is moving
only in one plane and in a straight line. Like if we roll a marble on a flat table, and if we roll it in
a straight line (not easy!), then it would be undergoing one-dimensional motion. There are four variables which put
together in an equation can describe this motion. These are
Initial Velocity (u); Final Velocity (v), Acceleration (a), Distance Traveled (s) and Time elapsed (t). The
equations which tell us the relationship between these variables are as given below.

v = u + at

s = ut + 1/2 at
2

Two and Three Dimensional Motion

Scalar or Vector?

To explain the difference we use two words: 'magnitude' and 'direction'. By magnitude we mean how much
of the quantity is there. By direction we mean is this quantity having a direction which defines it. Physical quantities
which are completely specified by just giving out there magnitude are known as scalars. Examples of scalar quantities are
distance, mass, speed, volume, density, temperature etc. Other physical quantities cannot be defined by just their
magnitude. To define them completely we must also specify their direction. Examples of these are velocity, displacement,
acceleration, force, torque, momentum etc.

Parallelogram law of vector addition

If we were to represent two vectors magnitude and direction by two adjacent sides of a parallelogram. The resultant
can then be represented in magnitude and direction by the diagonal. This diagonal is the one which passes through the
point of intersection of these two sides.

Resolution of a Vector

It is often necessary to split a vector into its components. Splitting of a vector into its components is called
resolution of the vector. The original vector is the resultant of these components. When the components of a vector are
at right angle to each other they are called the rectangular components of a vector.

Rectangular Components of a Vector

As the rectangular components of a vector are perpendicular to each other, we can do mathematics on them. This allows
us to solve many real life problems. After all the best thing about physics is that it can be used to solve real world
problems.

Note: We will show all
vector quantities in bold. For example A will be scalar quantity and
A will be a vector quantity.

Let
Ax and
Ay be the rectangular components of a vector
A

then

A =
Ax +
Ay
this means that vector
A is the resultant of vectors
Ax and
Ay

A is the magnitude of vector
A and similarly A
x and A
y are the magnitudes of vectors
Ax and
Ay

As we are dealing with rectangular components which are at right angles to each other. We can say that:

A = (A
x + A
y)
1/2

Similarly the angle Q which the vector
A makes with the horizontal direction will be

Q = tan
-1 (A
y / A
x)

Laws of Motion

Newton's laws of motion

Through Newton's second law, which states:
The acceleration of a body is directly proportional to the net unbalanced force and inversely proportional to the
body's mass, a relationship is established between

Force (F), Mass (m) and acceleration (a). This is of course a wonderful relation and of immense usefulness.

Knowing any two of the quantities automatically gives you the third !!

Momentum

Momentum (p) is the quantity of motion in a body. A heavy body moving at a fast velocity is difficult to stop. A light
body at a slow speed, on the other hand can be stopped easily. So momentum has to do with both mass and velocity.

Often physics problems deal with momentum before and after a collision. In such cases the total momentum of the bodies
before collision is taken as equal to the total momentum of the bodies after collision. That is to say: momentum is
conserved.

Impulse

This is the change in the momentum of a body caused over a very short time. Let
m be the mass and v and u the final and initial velocities of a body.

Work, Energy and Power

Work and energy

As we know from the law of conservation of energy: energy is always conserved.

Work is the product of force and the distance over which it moves. Imagine you are pushing a heavy box across the
room. The further you move the more work you do! If
W is work, F the force and x the distance then.

W = Fx

Energy comes in many shapes. The ones we see over here are
kinetic energy (KE) and potential energy (PE)

Transitional KE = ½ mv
2

Rotational KE = ½ Iw
2

here
I is the moment of inertia of the object (a simple manner in which one can understand moment of inertia is to
consider it to be similar to mass in transitional KE) and
w is angular velocity

Elastic PE = ½ k L
2

where
k is the spring constant ( it gives how much a spring will stretch for a unit force) and
L is the length by which the spring is stretched or compressed form the equilibrium position.

Power

Power (P) is work( W) done in unit time (t).

P = W/t

as
work and energy (E) are same it follows power is also energy consumed or generated per unit time.

P = E/t

In measuring power Horsepower is a unit which is in common use. However in physics we use Watt. So the first thing to
do in solving any problem related to power is to convert horsepower to Watts.
1 horsepower (hp) = 746 Watts

Circular Motion

In the diagram
v is the tangential velocity of the object.
a is the centripetal (acting towards the center of the circle) acceleration and F is the centripetal force.
r is the radius of the circle and m is mass of the object.

as
work and energy (E) are same it follows power is also energy consumed or generated per unit time.

a = v
2 / r

F = ma = mv
2/r

Gravitation

Kepler's Laws

Towards the end of the sixteenth century, Tycho Brahe collected a huge amount of data giving precise measurements of
the position of planets. Johannes Kepler, after a detailed analysis of the measurements announced three laws in 1619.

1. The orbit of each planet is an ellipse which has the Sun at one of its foci.

2. Each planet moves in such a way that the (imaginary) line joining it to the Sun sweeps out equal areas in equal
times.

3. The squares of the periods of revolution of the planets about the Sun are proportional to the cubes of their mean
distances from it.

Newton's law of universal gravitation

About fifty years after Kepler announced the laws now named after him, Isaac Newton showed that every particle in the
Universe attracts every other with a force which is proportional to the products of their masses and inversely
proportional to the square of their separation.

Hence:

If
F is the force due to gravity, g the acceleration due to gravity, G the Universal Gravitational Constant (6.67x10-11
N.m2/kg2), m the mass and r the distance between two objects. Then

F = G m
1 m
2 / r
2

Acceleration due to gravity outside the Earth

Here let
r represent the radius of the point inside the earth. The formula for finding out the acceleration due to gravity
at this point becomes:

g' = ( r / re )g

In both the above formulas, as expected,
g' becomes equal to g when r = re.

Properties of Matter

Density

The mass of a substance contained in unit volume is its
density (D).

D = m/V

Measuring of densities of substances is easier if we compare them with the density of some other substance of know
density. Water is used for this purpose. The ratio of the density of the substance to that of water is called the
pecific Gravity (SG)S of the substance.

SG = D
substance / D
water

The density of water is 1000 kg/m
3

Pressure

Pressure (P) is Force (F) per unit area (A)

P = F/A

Specific Heat

You may have noticed that metals, for example copper, heat faster than water. You would require 4186 J of heat to
raise the temperature of water by 1 degree Celsius. On the other hand 1 kg of copper would zoom to this temperature after
it receives only 387 J of heat. It is known that every substance has a unique value of amount of heat required to change
the temperature of 1 kg of it by 1 degree Celsius. This number is referred to as the specific heat of the substance. Let
Q be the heat transferred to m kg of a substance, thereby changing its temperature by dT. The specific heat c of
the substance is defined as

c = Q/mdt

Juggle the expression, and we get the heat transferred from a body to its surroundings or the other way around. This
is given by.

Q = m c dT

For example the heat required to increase the temperature of half a kg of water by 3 degrees Celsius can be determined
using this formula. Here m, mass of water is 0.5 kg and the dt, the temperature rise = 3 deg C and we know the specific
heat of water is 4186 J/kg. So here the heat required will be

Q = 0.5 x 4186 x 3 =6280 J

It is as simple as that !!

he table below gives the specific heat of some common substances

J/kg.
o C

cal/g.
o C

Aluminium

900

0.215

Copper

387

0.0924

Glass

837

0.200

Gold

129

0.0308

Ice

2090

0.500

Iron

448

0.107

Silver

234

0.056

Steam

2010

0.480

Water

4186

1.00

Electricity

Electricity

According to Ohm's Law electric
potential difference(V) is directly proportional to the product of the
current(I) times the
resistance(R).

V = I R

The relationship between
power (P) and current and voltage is

P = I V

Using the equations above we can also write

P = V
2 / R

and

P = I
2 R

Resistance of Resistors in Series

The
equivalent resistance (R
eq) of a set of; resistors connected in series is

R
eq = R
1 + R
2 + R
3 + - - -

Resistance of Resistors in Parallel

The
equivalent resistance (R
eq) of a set of resistors connected in parallel is